ITTO20120981A1 - METHOD AND PLANT FOR POWDER COATING OF ELECTRICALLY NON-CONDUCTIVE ELEMENTS, IN PARTICULAR BRAKE PADS - Google Patents

Description

DESCRIPTION

â € œMETHOD AND SYSTEM FOR POWDER PAINTING OF ELECTRICALLY NON-CONDUCTIVE ELEMENTS, IN PARTICULAR BRAKE PADSâ €

The present invention relates to a method and a plant for the powder coating of electrically non-conductive elements, in particular of brake pads made with friction materials of the NAO type (â € œNot Asbestos Organicâ €), but without components. metallic.

The friction materials used as linings in brake pads of drum brakes and as brake pads in disc brakes for vehicles and other equipment (e.g. clutch discs) are formed from a compound comprising a fibrous base, an organic binder (in generally a phenolic-based synthetic plastic resin) and a filler or filler (â € œfillerâ €). As a fibrous material, instead of asbestos, which has long been banned by legislation due to its danger to the environment, mixtures of organic and inorganic materials are used, such as rock wool, aramid and carbon fibers and metals, such as fibers or powder of copper, tin, iron, aluminum and other metals or metal alloys, such as bronze and brass. For example, EP1227262 teaches a friction material of the type described above, including about 10% by volume of copper fibers as fibrous material, from 0.1 to 15% by volume of tin and / or tin sulfide and from 4 to 9% by volume of bronze fibers.

However, in order to preserve the environment, brake pads made with metal-free and, therefore, electrically non-conductive compounds are increasingly required.

Although in brake pads the compound is formed as a plate which is in turn mounted on a metal base to form the actual pad, painting brake pads made with non-conductive compounds presents numerous problems. In particular, it is currently impossible to use the powder coating systems used today to paint brake pads made with compounds containing metals and, therefore, electrically conductive.

More generally, it is known that in order to powder paint (technology that requires the use of electrostatic charges) electrically non-conductive elements, such as mechanical parts made of plastic, conductive primers are used, which are applied to the surface to be painted of the non-conductive element. However, conductive primers are based on organic solvents, which are dangerous and polluting.

The attempts made to powder paint brake pads made with non-conductive compounds have so far been unsuccessful, as the painting, when it is successful (for example thanks to the presence of the metal support base of the plate in non-conductive compound) unsatisfactory coating thicknesses: the brake pads thus obtained are therefore not able to pass the usual corrosion resistance tests.

The only current alternative is therefore to resort to other painting technologies, which are however expensive and require bulky and purpose-built systems, thus requiring investments that are often unacceptable for current production volumes.

The purpose of the present invention is to provide a method and a system for the powder coating of electrically non-conductive elements, in particular of brake pads, which allows to obtain optimal spreading, coatings and surface finishing, completely comparable to those obtainable on current systems intended to treat brake pads made with electrically conductive compounds as they contain metals, all being able to use, with minimal modifications, the current systems and without the use of conductive primers.

The invention therefore relates to a method for powder coating of electrically non-conductive elements, in particular of brake pads, as defined in claim 1. The invention also relates to a powder coating plant for treating electrically non-conductive mechanical elements, in particular brake pads, according to claim 8.

According to the main aspect of the invention, upstream of an electrostatically charged painting powder deposit / deposition station and of a firing station for melting and polymerizing the painting powder to form a coating layer on a surface to be painted, electrically non-conductive elements there is a pre-treatment station to make the elements to be painted temporarily electrically conductive by uniformly moistening them by adsorption and / or deposition of poorly mineralized water on at least the aforementioned surface to be painted, to such an extent as to produce a measurable weight increase in electrically non-conductive elements, which makes these elements electrically conductive. The adsorbed and / or deposited water is subsequently eliminated in the cooking station.

Here and in the following, by â € œ poorly mineralized waterâ € is meant a water having a chemical-physical composition free of potentially aggressive ions for ferrous metals and a dry residue quantitatively comparable to that of a water with low mineral content.

In particular, according to one aspect of the invention, the poorly mineralized water to be used according to the method of the invention and in the plant to implement it must have a chemical composition such as to have a specific conductivity measured at 20 ° C of at least 150 S / cm [â € œSâ € ̈, in the International System, the symbol of Siemens].

Furthermore, as determined by the Applicant's technicians, it must be adsorbed and / or deposited on each electrically non-conductive element, to be made electrically conductive, in an amount such as to produce a weight increase of each electrically non-conductive element between 0, 15% and 0.30%.

In this way, the electrical resistance of brake pads made in metal-free NAO compounds goes from values of the order of 1,000,000 of Mâ „¦ (Mega Ohm) before pre-treatment, to values after pre-treatment , of the order of 0.011 Mâ „¦, and then go back to the pre-treatment values at the end of the cooking phase, which generally takes place in a tunnel oven. Therefore, pre-treated brake pads can be painted with traditional powder coating methods, charging the painting powders electrostatically, using the same cycles already in use for brake pads obtained with NAO compounds containing metals and, therefore, â € œof courseâ € electrically conductive.

According to further aspects of the invention, the plant for carrying out the method of the invention can be made according to two different embodiments.

In a first embodiment, the pre-treatment station comprises a superheated steam generator starting from poorly mineralized water, for example well water; a plurality of nozzles for delivering superheated steam towards the electrically non-conductive elements means of mixing steam / air capable of producing towards the electrically non-conductive elements, when these are on the transport device upstream of the electrostatic application station of the painting powders , an air / steam flow with an air-to-steam ratio between 15 and 30 m <3> / kg ;; and cooling means within which the electrically non-conductive elements are guided by the transport device downstream of the air / steam mixing means and immediately upstream of the electrostatic application station for the painting powders.

In a second preferred embodiment, the pre-treatment station comprises a rack with motorized rollers on which the electrically non-conductive elements pass neatly; a plurality of nozzles for delivering high pressure jets of poorly mineralized water towards the electrically non-conductive elements, said nozzles being able to create a mist all around the electrically non-conductive elements; at least one feed pump for the nozzles; and a suction hood arranged facing the dispensing nozzles.

According to this embodiment, the entire pre-treatment station has a longitudinal dimension, i.e. in the direction of advancement of the brake pads to be painted along the system as a whole, of only 45 mm (equal to the length of the rack with motorized rollers), carries out the pretreatment phase in just 10 seconds and with a completely negligible energy consumption (about 1 KW).

These results are obtained in particular by means of a double device: on the one hand, the nozzles are arranged aligned with each other under the rack with motorized rollers, so as to direct their jets from the bottom upwards, at a vertical distance. prefixed by the rack with motorized rollers, so that an empty space, possibly adjustable, is delimited between it and the nozzles; and, on the other hand, the extractor hood is defined by a tubular element equipped towards the nozzles with a straight through suction slot arranged parallel to the direction of alignment of the nozzles with each other. In fact, it has been found that these two devices, when present at the same time, have a synergistic effect that allows you to adjust with extreme precision the quantity of poorly mineralized water that moistens the compound of the brake pads being treated. This is essential in order to achieve the desired electrical conductivity without wetting the brake pads too much.

A further advantage of the system according to the invention is that it is able to paint brake pads made with both NAO metal-free compounds and â € œtraditionalâ € brake pads equipped with NAO compounds containing metals, simply by deactivating the station. pre-treatment. It is therefore possible to pass from one production to another on the same plant without making any changes.

Finally, the plant and the method of the invention are totally ecological.

Further characteristics and advantages of the present invention will appear clear from the following description of two of its non-limiting embodiments carried out purely by way of example and with reference to the figures of the annexed drawings, in which:

Figure 1 illustrates a longitudinal schematic view in elevation and partially in section of a powder coating plant made according to the invention;

- Figure 2 schematically illustrates, on an enlarged scale, a cross section of a component of the plant of Figure 1;

- Figure 3 illustrates on an enlarged scale a perspective view of a second component of the system of Figure 1;

Figure 4 1 illustrates a longitudinal schematic view in elevation and partially in section of a second embodiment of the powder coating plant made according to the invention; And

- Figures 5 and 6 illustrate two diagrams which provide the weight and electrical conductivity variations of NAO brake pads with metal-free compound treated according to the method of the invention.

With reference to Figure 1, 1 indicates as a whole a system for powder coating electrically non-conductive elements 2, in particular known brake pads and therefore not shown in their specific form but only, schematically, as blocks , in particular made with metal-free NAO compounds.

Such a type of brake pads is first produced by molding the compound at a temperature between 130 and 200 ° C to form plates, which are then cured by heat treatment, mounted on a ferrous metal support and finally painted together with the support.

In the case of metal-free compounds, however, the brake pads thus obtained, with current technology, cannot be powder-coated, but only by spray, with all the problems, including ecological ones, that this entails.

According to the invention, vice versa, the powder coating plant 1 is used, which generally comprises a pre-treatment station 3, in which the electrically non-conductive elements 2 are temporarily rendered electrically conductive, and are then â € œtransformedâ € into electrically conductive elements 2b, a station 4, of a known type and therefore illustrated in a purely schematic way, in which painting powders 5 are electrostatically applied on the elements 2b, a melting station 6 and polymerization of the painting powders 5, preferably defined by a tunnel baking oven of a known type and which has also been illustrated only in a purely schematic way for simplicity, and at least one transport device 7 adapted to make the electrically non-electrically transverse elements conductive 2 to be painted in series along stations 3, 4 and 6, to cross them and give rise, in the end, to and electrically non-conductive elements 2 provided, on at least one of their surfaces 8 to be painted (which can extend, depending on the case, over the entire external surface of the elements 2 or only part of it), with a coating layer 9 formed by the powders coating 5 melted and polymerized along the transit in the oven 6, at the usual temperatures required for powder coating processes (generally around 200-220 ° C).

In the illustrated non-limiting example, the system 1 comprises a transport device 7 which passes through the stations 4 and 6 and which places the painted elements 2 on a table 10, and a second device 7 which feeds the elements 2 to the station 3, located upstream of stations 4 and 6.

Here and below, the terms `` upstream '' and `` downstream '' must be understood as referring to a direction D of advancement of the electrically non-conductive elements 2 along the system 1, and, specifically, the stations 3, 4 and 6, indicated by the arrows in figure 1.

According to the invention, the pre-treatment station 3 comprises means 11 for depositing and / or adsorbing poorly mineralized water on at least the surface 8 to be painted of the electrically non-conductive elements 2 and, preferably, on each entire electrically non-conductive element 2, in measure such as to produce a weight increase (weight increase) measurable in the electrically non-conductive elements 2, which determines a retention on the same (at least in correspondence with the surface 8) of a quantity of poorly mineralized water such as to temporarily make these elements 2 electrically conductive , â € œtransforming themâ € into elements 2b.

It has already been said that â € œ poorly mineralized waterâ € means water having a chemical-physical composition free of ions potentially aggressive to ferrous metals (which make up the support of the NAO compound plates of the brake pads 2) , therefore free of ions such as HCO3-, and a dry residue quantitatively comparable to that of a water with low mineral content. The poorly mineralized water usable in the plant 1 according to the invention must however have a chemical composition such as to have a specific conductivity measured at 20 ° C of at least 150 S / cm.

Furthermore, again according to the invention, the melting and polymerization station 6 of the painting powders 5 must be suitable not only to melt and polymerize the powders 5, but also to eliminate by evaporation at least part of the (in fact substantially all) of the water previously adsorbed and / or deposited on the electrically non-conductive elements 2. Taking into account that station 6 consists of a known type tunnel kiln in which temperatures of the order of 200 ° C are reached, this last characteristic is intrinsically present. However, this characteristic, necessary for the purposes of the invention, excludes the use of melting and polymerization stations where technologies are used that do not safely lead to the elimination of the water retained on / in the elements 2.

According to the embodiment illustrated in Figure 1, the pre-treatment station 3 of the plant 1 comprises, in combination: a rack 12 comprising a plurality, for example four, of motorized rollers 13, arranged apart from each other in the direction D and on which the electrically non-conductive elements 2 transit in order in use; a plurality of nozzles 14 for delivering high pressure jets 15 of poorly mineralized water (as defined above, for example well water) towards the electrically non-conductive elements 2 which are located on the rack 12; at least one pump 16 for feeding the water (at a pressure greater than 60 bar and preferably equal to 70 bar) poorly mineralized (contained in a tank not illustrated for simplicity or directly taken from a well or other natural source) to the nozzles 14 ; and an aspiration hood 17 arranged facing the delivery nozzles 14 and served by a vacuum aspirator 18 which sucks the ambient air together with a large part of the water delivered by the nozzles 14, so as to avoid water droplets falling on the elements 2 after the jets 15 have passed through the rack 12 (through the spaces present between the rollers 13) and the elements 2 present on the rollers 13 are `` wet ''.

In particular, the nozzles 14 emit jets 15 which widen into a cone, dispersing the delivered water in the ambient air, thanks to the pressure drop at the outlet, to create all around the electrically non-conductive elements 2 in transit on the rollers 13, around the rollers 13 themselves and, more generally, throughout the volume of space between the nozzles 14 and the hood 17, a mist (i.e. a very fine and dense dispersion of micro-drops of water in the air), which laps and it is deposited in a conductive film on the insulating surface of the elements 2 to be then sucked, for the excess part, into the hood 17 by means of the aspirator 18, which sends it into an exhaust duct 19. Being a simple mixture of water and air, the mist sucked in by the hood 17 can be discharged directly into the environment, or treated for the recovery, at least partially, of the water.

According to the preferred embodiment illustrated, the dispensing nozzles 14 (see also Figure 3) are arranged aligned side by side, spaced apart, according to an alignment direction L (Figure 3) which is transverse to the direction of advancement D of the elements 2 electrically non-conductive along the plant 1, in general, and on the rack 12 with motorized rollers 13, in particular. The L direction is specifically perpendicular to the D direction.

In the illustrated example, the nozzles 14 are arranged below the rack 12 with motorized rollers 13, so as to direct the jets 15 from the bottom upwards; the nozzles 14 are furthermore located, according to an important aspect of the invention, at a predetermined vertical distance T from the rack 12 with motorized rollers 13, so that between the rollers 13 of the rack 12 and the nozzles 14 an empty space is delimited, indicated by S in figure 3.

Preferably, the station 3 of the plant 1 also comprises means 20 for varying the predetermined vertical distance T in an adjustable way, illustrated only schematically in Figure 3 as holes 20b and as a hydraulic or pneumatic piston 20c (but any other type of actuator Is eligible). According to what is not limitedly illustrated in Figure 3, the rollers 13 each terminate with a gear 21, which is rotated by a gear transmission 23 by means of a motor 24 common to all four rollers 13, which thus rotate in a synchronous. Rollers 13, transmission 23 and motor 24, together with all the remaining parts of the rack 12, are supported by a frame 25 forming part of the support structure of the plant 1, not shown for simplicity.

The frame 25 also comprises upright supports 26 possibly provided with vertically spaced holes 20b; on the upright supports 26 (only one of which is illustrated for simplicity in figure 3, but it is clear that at least two are needed, one for each side of the rack 12), for example, runs a C-shaped profile 27 which supports a tube 28 oriented in the direction L and carrying the nozzles 14 at the top. These are arranged flush with a pair of bulkheads 29 also integral with the section 27 and an upper edge of which defines the distance T of the nozzles 14 from the rollers 13. profiled27 can be activated automatically by the actuator means 20c, or moved by hand when the system 1 is stationary and fixed in the new position by means of the holes 20b, into which special fixing pins or cleats, of a known type and not illustrated for simplicity, are introduced. .

Together with the characteristics described above, the extractor hood 17 is not realized as the one illustrated only schematically in Figure 1, but is instead realized as a extractor hood 17b according to what is illustrated in Figure 2.

The hood 17b is defined by a tubular element 30 equipped towards the nozzles 14, therefore at the bottom, with a straight through slit 31 arranged parallel to the alignment direction L of the nozzles 14. Said rectilinear slit 31 is delimited longitudinally by a pair of edges 32 folded in a V shape towards the inside of the tubular element 30 so as to delimit inside and below the same a siphon 33 for collecting water, from which siphon 33 the collected water ( which in this way does not fall back towards the elements 2) is then sucked for â € œbubblingâ € by the aspirator 18. For this purpose, the aspirator 18 is arranged in a position diametrically opposite to the slot 31.

In this way, the fog produced by the nozzles 14, thanks to the presence of the empty space S between them and the rollers 13, in which the empty space S the jets 15 can be dispersed, forming precisely the aforementioned fog, is sucked by the hood 17b through the slot 31, making them follow an obligatory path around the elements 2 and through the rack 12. Subsequently, the droplets of water suspended in the ambient air to form the aforementioned fog, are sucked into the hood 17b together with the air in which are suspended; most of it ends up directly in the exhaust 19; the water droplets that do not end up at the drain 19 lose speed within the pipe 30, which functions as a sort of cyclone, and are collected inside the siphon 33, without falling back onto the elements 2, also thanks to the presence of the folded edges 32.

Consequently, the elements 2 in transit on the rack 12 receive a controlled quantity of water, which they consider, increasing, even if slightly, in weight. System 1, and in particular the elements making up station 3, is / are sized so that the weight increase of elements 2 when they pass through station 3 is between 0.15% and 0 , 30%.

With reference now to figure 4, where the details similar or identical to those already described are indicated for simplicity with the same numbers, it shows a system 1b which represents a possible variant of the system 1 according to the invention, previously described.

Plant 1b differs from plant 1 in that the pre-treatment station 3 is replaced by a pre-treatment station 3b which always has the purpose of moistening uniformly and in a controlled manner, by adsorption and / or deposition of poorly mineralized water, at least the surface 8 to be painted and, preferably, each entire electrically non-conductive element 2, but does so in a different way.

The pre-treatment station 3b comprises: a superheated steam generator 35 which receives a stream F of poorly mineralized water, as defined above, and heats it to form a stream V of steam at about 200 ° C; a plurality of nozzles 38 for delivering said steam towards the electrically non-conductive elements 2; and mixing means 37 steam / air adapted to produce towards the electrically non-conductive elements 2, when these are on the transport device 7, an air / steam flow with an air-steam ratio between 15 and 30 m <3> / kg .

According to the illustrated non-limiting example, the system 1b comprises three different transport devices 7, a first to feed the elements 2 to the station 3b, a second which is an integral part of the station 3b and on which the elements 2 are located when they are hit by the air / steam flow, and a third passing through stations 4 and 6 to then deposit the painted elements 2 on table 10. The nozzles 38 (as well as the entire station 3b) are obviously arranged upstream of the station for electrostatic application 4 of painting powders 5 and are configured according to a scheme similar to that of the nozzles 14, but are arranged in use above the elements 2 and above the transport device 7 on which the elements 2 are neatly arranged as they pass in station 3b.

The station 3b also comprises cooling means 40 for the elements 2b arranged immediately upstream of the powder coating station 4. The elements 2b are none other than the non-conductive elements 2 which have retained the weakly mineralized water delivered through the aforementioned air / steam flow in a controlled way and which therefore forms (as also in the case of the plant 1 described above) a covalent bond directly above the elements 2, which thus become the electrically conductive elements 2b.

In the illustrated non-limiting embodiment example, the air / steam mixing means 37 and the cooling means 40 are defined by two adjacent portions 390 and 391 of a tunnel hood 39 arranged above the transport device 7 which is part of the station 3b at a predetermined distance T2 from it and from a vacuum fan / aspirator 392 mounted above the tunnel hood 39 and which draws ambient air into the tunnel hood 39 through an opening or gap 393 defined by the distance or empty space T2 included / or between the upper surface of the transport device 7 on which the elements 2 / 2b lie and the lower edge of the tunnel hood 39; this air, together with a large part of the steam delivered by the nozzles 38, which are arranged inside the tunnel hood 39 inside the portion 390 distal from the painting station 4, is eliminated through a drain 394. The portion 391 of the hood a tunnel 39, on the other hand, is close to station 4 and is immediately downstream of the nozzles 38 and immediately upstream of station 4 for the electrostatic application of the painting powders 5.

In the portion 390 the steam V delivered by the nozzles 38 mixes with part of the ambient air sucked through the port 393 inside the hood 39 and the air / steam flow thus formed and directed towards the fan 392 laps the elements 2, depositing on the themselves a quantity of poorly mineralized water, which remains adherent to the surface 8 with a covalent bond â € œtransformingâ € the non-conductive elements 2 into conductive elements 2b. In the portion 391, the remaining part of the ambient air sucked into the tunnel hood 39 by the fan 392 touches the electrically conductive elements 2b eliminating any excess water from them and cooling them (the elements 2 / 2b have in fact heated up in the portion 390 due to the effect of the heat released by the vapor V at a temperature not suitable for carrying out the electrostatic deposition of the powders 5).

On the basis of what has been described, it is clear that both systems 1 and 1b are suitable for carrying out a method for powder coating electrically non-conductive elements 2, in particular brake pads, comprising: a pre-treatment phase, in wherein the electrically non-conductive elements 2 are made electrically conductive on at least one surface 8 to be painted thereof, giving rise to the temporarily conductive elements 2b; a step of depositing an electrostatically charged painting powder 5 on the surface 8 to be painted; and a firing step, in which the painting powder 5 previously deposited on the elements 2b is melted and polymerized to form a coating layer 9 on the surface 8 to be painted.

According to the invention, the pre-treatment phase, instead of consisting in the spray deposition and with the use of solvents of a conductive primer, as in the known art, consists in wetting uniformly by adsorption and / or deposition of poorly mineralized water, as defined above, at least the surface 8 to be painted and, preferably, each entire electrically non-conductive element 2, to such an extent as to produce a measurable weight increase in the electrically non-conductive elements 2, which temporarily renders these elements 2 electrically conductive , giving rise to elements 2b.

The water held by the elements 2 electrically non-conductive, to form the elements 2b, which differ from the elements 2 precisely by the presence on at least the surface 8 of a film of poorly mineralized water valently bound to the surface 8, is subsequently eliminated at least partly (preferably substantially completely) during the cooking step.

The poorly mineralized water that is covalently retained by the elements 2 in the pre-treatment phase must have a chemical composition such as to have a specific conductivity measured at 20 ° C of at least 150 S / cm. Furthermore, the pre-treatment step is carried out in such a way that on each electrically non-conductive element 2 a quantity of water (adsorbed and / or deposited) is retained such as to produce a weight increase of each electrically non-conductive element 2 between 0, 15% and 0.30%.

By means of plant 1b, the pre-treatment phase is carried out by producing, starting from the aforementioned poorly mineralized water, superheated steam at at least 200 ° C, mixing a flow V of this superheated steam with at least part of a flow A of air sucked into the tunnel hood 39 from the external environment to generate together with the nozzles 38 housed in the portion 390 of the tunnel hood 39 an air / steam flow with an air-steam ratio between 15 and 30 m <3> / kg, and directing this flow of steam / air, by means of the aspirator 392 which is arranged in correspondence with the portion 391 of the tunnel hood 39 and, therefore, moved in a position close to the painting station 4 and arranged downstream of the nozzles 38, on the electrically non-conductive elements 2, arranged neatly in series on a conveyor belt (device 7). This pre-treatment phase is immediately followed by a cooling phase of the electrically non-conductive elements 2 and made electrically conductive (elements 2b) by covalent retention of poorly mineralized water. This cooling step is carried out before the execution of the deposit step of the painting powder 5, in the portion 391 of the tunnel hood 39, arranged downstream of the portion 390 in which the nozzles 38 are located.

By means of the plant 1, on the other hand, the pre-treatment phase is carried out by spraying high pressure jets 15 of poorly mineralized water on the electrically non-conductive elements 2 in order to create a fog all around the latter, and at the same time sucking up the fog by means of the extractor hood 17 / 17b.

In this case, the jets 15 of said poorly mineralized water are directed onto the electrically non-conductive elements 2 from bottom to top, while the elements 2 move on a rack 12 with motorized rollers, above which the extractor hood is arranged. 17.

The effects of the pre-treatment method according to the invention have been experimentally verified on a series of samples. A plurality of known types of brake pads have been produced but using metal-free NAO compounds.

A part of these brake pads are treated by the station 3b previously described with vapor / air mixture and subsequent cooling, measuring their weight before treatment, after contact with the air / vapor mixture (wet pads) in the 390 portion of the tunnel hood 39, after cooling in the portion 391 of the tunnel hood 39, and after cooking in the tunnel oven 6, at the same temperature used to treat traditional painted tablets. The results obtained on different samples are shown in figure 5. As can be immediately appreciated, the diagrams illustrating the weight variation of the different samples are completely comparable as a trend and show an increase in weight of the wet tablets, which decreases after the cooling and which substantially disappears at the end of the cooking phase, highlighting how the water retained by the brake pads after dispensing the steam / air mixture is eliminated in the painted pads. Conversely, the solid line graph shows the trend of the weight variation of a painted pad: as can be seen, the weight of the pad after cooling remains constant, since the weight of water lost corresponds to the weight of the paint (of the powder 5 painting) laid and fired in stations 4 and 6.

On the other hand, another part of the brake pads are treated by means of the station 3 described above, observing the same weight variation trend.

Finally, the average of the measured weight increase / decrease and electrical resistance values of the â € œwhiteâ € (unpainted) brake pads are reported before treatment with water, after treatment, at the exit of the tunnel hood 39 and at the ™ exit from the oven 6. The result obtained is shown in the graphs of figure 6. As can be seen, in the â € œwetâ € state, the non-conductive brake pads become electrically conductive, presenting average electrical resistance values between 0.01 and 0 , 02 Mega Ohm, against the average electrical resistance values of the untreated (not wet) tablets and after baking in the oven 6 of about 1,000,000 Mega Ohms.

Since stations 4 and 6 are identical to those of the â € œtraditionalâ € painting systems intended to treat brake pads obtained with electrically conductive compounds (containing metals), it is clear that systems 1 and 1b can be derived from existing systems, simply adding station 3 / 3b in series on the system. Furthermore, the systems 1 and 1b, simply by activating / deactivating the stations 3 and 3b, are suitable for treating both brake pads obtained with conductive compounds and brake pads obtained with non-conductive compounds.

The objects of the invention are therefore fully achieved.

Claims (12)

CLAIMS 1. Method for the powder coating of electrically non-conductive elements (2), in particular brake pads, comprising a pre-treatment step, in which the electrically non-conductive elements (2) are made conductive on at least one surface (8 ) to be painted, a deposit phase of an electrostatically charged painting powder (5) on the surface to be painted, and a firing phase, in which the painting powder (5) is melted and polymerized to form a coating layer (9) on the surface to be painted; characterized by the fact that the pre-treatment phase consists in uniformly moistening at least the surface (8) to be painted and, preferably, each entire electrically non-conductive element (2), uniformly by covalent retention of poorly mineralized water, to such an extent as to produce a measurable weight increase in the electrically non-conductive elements (2), which makes these elements electrically conductive (2b); the water retained on the electrically non-conductive elements being subsequently at least partially eliminated during the cooking phase. 2. Method according to claim 1, characterized in that the poorly mineralized water that is retained in the pre-treatment phase must have a chemical composition such as to have a specific conductivity measured at 20 ° C of at least 150 S / cm . 3. Method according to claim 1 or 2, characterized in that in the pre-treatment step an amount of water is made to retain on each electrically non-conductive element (2) such as to produce a weight increase of each electrically non-conductive element comprised between 0.15% and 0.30%. Method according to one of the preceding claims, characterized in that the pre-treatment step is carried out by producing, starting from said poorly mineralized water, superheated steam at at least 200 ° C, mixing a flow (V) of said superheated steam with an air flow (A) to generate an air / steam flow with an air-to-steam ratio between 15 and 30 m <3> / kg, and directing this steam / air flow onto the electrically non-conductive elements, neatly arranged in series on a conveyor device (7); said pre-treatment step being followed by a cooling step of the electrically non-conductive elements made conductive (2b) by covalent retention of poorly mineralized water. 5. Method according to claim 4, characterized in that said pre-treatment step is performed by delivering the steam flow (V) by means of a plurality of nozzles (38) housed within a first portion (390) of a tunnel hood ( 39) arranged above said conveyor device (7) and at the same time sucking ambient air into the tunnel hood (39) by means of a fan (392) arranged in correspondence with a second portion (391) of the tunnel hood (39) arranged downstream of the first portion (390); in the second portion (391) of the tunnel hood (39) the cooling step of the electrically non-conductive elements is carried out due to the suction of ambient air inside the tunnel hood (39). Method according to one of claims 1 to 3, characterized in that the pre-treatment step is carried out by spraying high pressure jets (15) of said poorly mineralized water on the electrically non-conductive elements (2) so as to create everything around the latter a fog, and at the same time aspirating said fog by means of a suction hood (17; 17b). 7. Method according to claim 6, characterized in that the jets (15) of said poorly mineralized water are directed on the electrically non-conductive elements (2) from bottom to top, while the elements (2) move on a rack (12) with motorized rollers (13), above which said suction hood (17; 17b) is arranged. 8. Plant (1; 1b) for the powder coating of electrically non-conductive elements (2), in particular brake pads, comprising a pre-treatment station (3; 3b), in which the electrically non-conductive elements are temporarily made electrically conductive (2b), a station (4) for the electrostatic application of painting powders (5), a station (6) for melting and polymerizing the painting powders (5), preferably defined by a tunnel baking oven, and at least a transport device (7) adapted to make the electrically non-conductive elements (2) to be painted to pass in an orderly manner along said stations (3; 3b, 4,6); characterized in that the pre-treatment station (3; 3b) comprises means (11) for covalently retaining poorly mineralized water on at least one surface (8) to be painted of said electrically non-conductive elements and, preferably, on each entire element electrically non-conductive, to an extent that produces a measurable weight increase in the electrically non-conductive elements, which makes these elements electrically conductive (2b); said station (6) for melting and polymerizing the painting powders being also able to eliminate by evaporation at least part of the water previously retained on the electrically non-conductive elements (2). Plant (1b) according to claim 8, characterized in that the pre-treatment station (3b) comprises, in combination: a superheated steam generator (35) starting from said poorly mineralized water; a plurality of nozzles (38) for delivering a flow (V) of said superheated steam towards the electrically non-conductive elements; steam / air mixing means (37) able to produce towards the electrically non-conductive elements (2), when these are on said at least one transport device (7), an air / steam flow with an air-vapor ratio between 15 and 30 m <3> / kg; and cooling means (391; 392) within which said electrically non-conductive elements are guided by said at least one transport device (7) downstream of said air / steam mixing means (37) and immediately upstream of the station (4 ) of electrostatic application of painting powders. Plant (1) according to claim 8, characterized in that the pre-treatment station comprises, in combination: a rack (12) with motorized rollers (13) on which said electrically non-conductive elements (2) neatly pass; a plurality of nozzles (14) for delivering high pressure jets (15) of said poorly mineralized water towards the electrically non-conductive elements (2), said nozzles (14) being able to create a fog; at least one feed pump (16) for said nozzles; and a suction hood (17; 17b) arranged facing said dispensing nozzles (14). 11. Plant according to claim 10, characterized in that, in combination: i) - said dispensing nozzles (14) are arranged aligned side by side with an alignment direction (L) transversal to a direction (D) of advancement of the electrically non-conductive elements on said rack (12) with motorized rollers; ii) - the delivery nozzles (14) and the at least one feed pump (16) are able to generate said jets (15) of water at a pressure of at least 60 bar; iii) - the nozzles (14) are arranged under the rack (12) with motorized rollers, so as to direct said jets (15) from the bottom upwards, at a predetermined vertical distance from the rack (12) to motorized rollers, so that an empty space (S) is delimited between this and said nozzles; iv) - the extractor hood (17b) is defined by a tubular element (30) equipped towards the nozzles with a straight slit (31) through the aspiration system arranged parallel to the alignment direction (L) of the nozzles. 12) Plant according to claim 11, characterized in that it comprises means (20) for varying said predetermined vertical distance (T) in an adjustable way; and by the fact that said straight slot (31) is delimited longitudinally by a pair of edges (32) folded in a V shape towards the inside of the tubular element so as to delimit inside, and below, itself a siphon (33) for collecting water.